How long can they go?

Hello, I'm currently on my second grow. I have decided to try some LST and topping. I'm starting to wonder now, if I might have did it too early. The plant on the right is a Zkittlez and the one on the left is a Strawberry Banana. The Zkittlez is on day 57 veg and the Strawberry is on day 49 veg. My question is how much longer can they go before I'm starting to see preflowers? I figured I would wait to switch nutes until than. Thanks and happy growing!